1) Field of the Invention
This invention relates to a drill string member and particularly to a drill string member arranged to alleviate or prevent differential sticking downhole.
2) Description of the Related Art
It is known from the Society of Petroleum Engineers, Paper SPE22549, “Differential Sticking Laboratory Tests can improve Mud Design” by M. Bushnell-Watson and S. S. Panesar, presented to the 66th Annual Conference and Exhibition of the Society of Petroleum Engineers held in Dallas, Tex., Oct. 6–9, 1991, that differential sticking occurs when a drill pipe, or logging tool, becomes embedded in mud filter cake and where the drill pipe or filter tool is held by the mud overbalance pressure. Once sticking has occurred, a large force is required to free the drill pipe, even if the mud overbalance is removed. Such sticking causes several hours of rig time being spent in attempting to free the drill pipe. In severe cases, the drill pipe cannot be freed and the well has to be sidetracked or abandoned. In the disclosure, a laboratory method is disclosed for freeing differentially stuck pipes with sheer and changes are proposed to the mud chemistry. As disclosed in the Society of Petroleum Engineers, Paper SPE14244, “A New Approach to Differential Sticking” by J. M. Courteille and C. Zurdo, presented at the 60th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers held in Las Vegas, Nev., Sep. 22–25, 1985, differential sticking has a high risk of occurrence in deviated wells and the paper describes recording the pressure at different points of the pipe/cake and cake/formation interfaces in a laboratory device. It is also known from, for example, U.S. Pat. No. 4,811,800 to produce a flexible drill string member for use in directional drilling in which the member has a spirally-shaped outer surface so as to make the member more flexible at traversing bends in boreholes. Such a member is formed from a steel tube which has the outer surface thereof machined to form a spiral. The wall thickness, therefore, varies.
U.S. Pat. No. 6,012,744 discloses a heavy weight drill pipe which also incorporates tubular members having spirally formed external surfaces and the spirally formed members are taught to reduce the chances of differential pressure sticking of the pipe when the pipe is used in a high angle or horizontal well bore.
Forces involved in the occurrence of differential sticking are substantially proportional to the area of a drill string element embedded in the filter cake. It will be understood by those skilled in the art that the filter cake is formed on the borehole wall when drilling through permeable formations. A reduction in the area of contact between the filter cake and the drill string element is thus a major objective of the spirally formed external surface of the member disclosed in U.S. Pat. No. 6,012,744.
By using a spiral formation with a right hand thread, in high angle holes, cuttings may be lifted into the main stream of the flowing mud and such right hand spiral designs increase the load effective at the bit by “screwing” the string towards the bit end while cuttings that have not been lifted into the main stream mud are pushed upwards along the low side of the hole in the manner of an Archimedian screw pump.
Such spirally formed members of the prior art have internal cross-sections which are cylindrical and with an outer surface of varying diameters which vary along the drill string longitudinal axis. The manufacturing process of drill string members such as drill pipes, intermediate weight drill string elements and heavyweight drill pipe elements, as well as drill collars having a non-circular cross-section for at least a part of their axial length, requires costly and time-consuming external removal of metal by milling. In addition, eventual drilling a long cylindrical bore may also be required when using an initially solid bar stock to produce the above-mentioned devices.